Tag containing '&' breaks filtering

[alisonrclarke]

We have a tag containing '&', and when selecting it to filter the radar, the ampersand is not escaped in the query string, so is treated like 2 query parameters, and no results are returned.

e.g. a tag X&Y when selected should give a url ending in ?tag=X%26Y but instead gives ?tag=X&Y. Note that the escaped URL works as expected.